The question is, would Eternity's "Everywhere at once" work when he is part of the sideline?

So he's definitely part of the starting force.

But this last quote seems to mean while he is part of the sideline his "Everywhere at once" would not be referenced? So he would not gain the Eternal keyword, and also wouldn't even be able to be put on the Sideline without some other reason to do so?

This is correct, he would not gain any keywords when added to the Sideline.

There is no way I can think of to add him to the sideline but if there were what makes you think he wouldn't gain keywords?

How are you coming to that conclusion?

Edit: To clarify, the quote makes it pretty clear that if he is on the sideline, then he is not part of your starting force.
As he is not part of your starting force, then he doesn't really count for the "other" part of his effect.

Oh, I just read it wrong. Hmmm, not noticing the word "not" makes a significant difference.

I was hung up on his bit about "other characters on your starting force" and if the "other" implied that he too would have to be on his starting force, which it doesn't explicitly but could be argued due to precedence, and I saw that quote, read it wrong, and thought, "Oh good, that makes it a non issue". So I was both wrong and needlessly confusing.

I could have sworn that there was a rule somewhere stating that characters' game effects do not work while they are off the map unless otherwise stated, but I can't find that readily right now.

I'm sure it has to exist, otherwise if you could get someone with "Other characters can't use ____." on your sideline, then that would apply. I'm just not finding it right now.

It seems like the 3rd quote in the original post covers you there, does it not?

You can only use/reference the parts of them that justify why they are on your sideline to begin with.

Not sure how Eternity is ending up on the Sideline in the first place

Going with it though... the wording is a little odd but people have quoted all of the relevant rules text for something on your Sideline not being part of your Starting Force or activating "during force construction" effects.

The part about game effects is in the Comprehensive Rulebook on page 2 but does not really apply here:

I'm pretty sure he wanted to use him with Uni-Mind.
If he could've picked up the Eternals keyword, then he'd be sideline material for Uni-Mind.

no love for Animal Man anymore? If this worked, he could do it also.

Ohhhhh. I am dumb.

I guess you could also use him as a SHIELD Level 7 ID Call-in assuming this worked (This would not work, please do not take it as me saying that!)

Yeah the Eternity/Uni-Mind/ID Card thing was covered pretty much as soon as Eternity was revealed. WK's didn't even bother answering the question on the WIN, they just sent rejection emails quoting the "Sideline Active" entry in the rules.

Yeah, this question came up in the Dials thread too. No "sideline active" label was pointed out.